Since I enabled video filters in smplayer with vdpau (for xvid/divx videos). X crashes more often.
IIRC smplayer was not runing at the time of the crash.
Even without video filters, moving a window with runing video leads to a high system cpu load and is very slow. Although I am not sure if this is the cause of the crash.

Now I had another crash while watching a movie, but now I had gdb running:
Program received signal SIGSEGV, Segmentation fault.
0x00007f50841533bd in ?? () from /usr/lib64/xorg/modules/drivers/nvidia_drv.so
(gdb) bt
#0 0x00007f50841533bd in ?? ()
from /usr/lib64/xorg/modules/drivers/nvidia_drv.so
#1 0x00007f50841542a0 in ?? ()
from /usr/lib64/xorg/modules/drivers/nvidia_drv.so
#2 0x00007f5084154646 in ?? ()
from /usr/lib64/xorg/modules/drivers/nvidia_drv.so
#3 0x000000000044bdfd in FreeWindowResources (pWin=0x7ffe6d0) at window.c:847
#4 0x000000000044def3 in DeleteWindow (value=0x7ffe6d0, wid=1166016855)
at window.c:926
#5 0x0000000000455790 in FreeResource (id=1166016855, skipDeleteFuncType=0)
at resource.c:556
#6 0x000000000042d83b in ProcDestroyWindow (client=0x5102330)
at dispatch.c:745
#7 0x00000000004a7497 in PanoramiXDestroyWindow (client=0x5102330)
at panoramiXprocs.c:256
#8 0x000000000042feec in Dispatch () at dispatch.c:439
#9 0x000000000042271c in main (argc=8, argv=0x7d9a10,
envp=<value optimized out>) at main.c:285